ON HOW TO BEST INTRODUCE THE CONCEPT OF DIFFERENTIAL IN PHYSICS
R. López-Gay, J. Martínez-Torregrosa, Dpto. de Didáctica General y Didácticas Específicas,
Alacant, Spain
A., Gras-Martí, Dpto. de Física Aplicada, Universitat d’Alacant, Alacant, Spain
G. Torregrosa, Dpto. de Análisis Matemático y Didáctica de las Matemáticas, Alacant, Spain
1. Introduction
The almost unanimous opinion of 103 high school Physics teachers, and the results of the analysis
of 38 Physics textbooks, indicate clearly that Differential Calculus is necessary to teach Physics in
the last years of high school, and it becomes indispensable at university level (Lopez-Gay et al.,
2001). This necessity is obvious since Differential Calculus allows to study physical situations with
a higher degree of complexity than the ones dealt with in elementary courses.
In agreement with this importance, it should be expected that the use with comprehension of
Differential Calculus would be a concern of Physics teachers and a result of the learning process;
and that it should be perceived as an indispensable tool to progress in the comprehension of
Physics. In spite of this, our teaching experience has showed us that the reality of the customary use
of Differential Calculus in Physics lectures is very different: textbooks and lecturers use it in an
operational and mechanical way, the students don’t understand what is being done and why is it
done when it is used even in simple situations, both teachers and students have low expectations of
it being used with comprehension and autonomy, its use is perceived as an obstacle and a source of
reject against Physics. This impression is confirmed by our analysis of the usual teaching practice.
A clear uneasiness among Physics teachers exists, for they feel obligated to use Differential
Calculus in their lectures, but are aware that their students don’t understand the reason and the
meaning of what is being done. Even worse, there is a diffuse uneasiness as a result of their
incapability to identify clearly the cause of this situation and to find an adequate way to overcome
it.
In order to confront this diffuse uneasiness, we have carried out a historical and epistemological
study (Martínez-Torregrosa et al., 2001) looking for answers to the following questions: in what
kind of situations is Differential Calculus used?, which is the global strategy of Calculus to deal
with those situations?, which is the meaning of the different concepts used?, which is the
relationship among them? These questions have remained concealed in the customary education
practice under a stream of rules and algorithms, which shows a clear disdain towards the more
understandable and conceptual aspects (Artigue and Viennot, 1987; Ferrini-Mundy and Gaudard,
1982, NCTM, 2000).
Taking into account that differential expressions constitute the starting point in the reasoning and
mathematisation of the physical situations, we have centred our study in the concept of differential,
although without losing sight of the global strategy of Calculus. The conclusions we have obtained
have enabled us to understand better the present deficiencies and to introduce proposals to
overcome them.
2. Towards a better understanding of the differential in physics
The concept of differential has not always answered to the same definition, nor has played the same
part in the whole of Calculus. In its origin, in the 17th century, the differential was invented to
undertake the step from approximation to exactness. For this, the differential of a quantity (dy) was
considered as an approximation of the increment of that same quantity (∆y), but an increment so
small that they tended to be the same (dy = ∆y). This way one could ignore the error made when the
(very small) increments were substituted by differentials, or when the terms containing differentials
were neglected (which L’Hôpital resumed in the equation: y+dy = y).
An analysis of 38 Physics textbooks shows that this concept of the differential -and the arguments
that accompany it- is the only one that appears in Physics lectures. The same criticisms that were
made more than 300 years ago about the use of infinitely small quantities, can be formulated
nowadays almost in the same terms: how small is the differential?, is it a fixed quantity, or a
variable quantity?, is it really possible to obtain an exact result by neglecting terms that are not
zero?, what criteria should be used to determine the differential expression in each specific case?
In general, in order to write the differential expression, an approximate expression of the increment
is written, with the certainty that, when that increment is very small, it can already be considered an
exact expression. For instance, it is said that the variation of the intensity of a plane wave when
crossing a material medium of very small thickness is: dI = -α·I·dx. But this is not true, no matter
how small dx or dI are. It is true, however, that when dx is very small, that expression is very
approximately –although it never coincides with- ∆I, but the same happens to other expressions (for
example: dI = -α·I2·dx, dI = -α·dx/I, dI = -α·I·dx2...). If dI never gets to coincide with ∆I, an error
will always exist when substituting one for the other. How then get to the exact result? Furthermore,
what criteria do we use to select which starting expression is adequate?
In usual teaching these questions are not dealt with. The wrong belief is implied that, whatever the
form of the approximate increment, as it is so small, the error will always be practically zero and
will not affect the result. This is wrong: there is only one correct differential expression for each
situation1. Therefore, the criticisms made to the use of the differential as an infinitely small quantity
do not only affect formal or philosophical questions, but have practical consequences.
Why are not wrong results obtained more frequently then? On the contrary, the use of Differential
Calculus is considered almost as a guarantee that the result is more correct than when it is not used?
It is known that by recalling past examples one selects the correct results, and the absurd results that
have historically been obtained are soon forgotten (Orton, 1983; Schneider, 1991). In teaching, the
starting differential expression is accepted as a dogma, without arguments nor analysis that justify
that particular choice and not a lot of other possible ones. This gives rise to the feeling that the
simple idea of differential as a very small increment, where everything is allowed, is enough to
sustain the whole of Calculus in the physical applications.
The weight of these criticisms and the adverse results lead to the creators of Calculus to doubt of the
identification between the differential and an infinitely small quantity [Kline, 1980, p. 480 and 511].
Leibniz left the differential without a rigorous meaning but, convinced of its utility, did not give up
the use of the differential: “the differentials may be used as an instrument, in the same way that the
algebraists use the imaginary roots with great benefit” [Kline, 1980, p. 509]. We have confirmed
this same ambiguity when we study in depth the significance that teachers and students assign to the
differential expressions they constantly use in the Physics lectures.
The following 150 years showed the enormous power of Calculus, but no substantial progress was
made in understanding correctly what was being done: one worked “in an almost blind way, often
guided by a naïve intuition that what they did had to be valid” (Eves, 1981). Early in the 19th
century Cauchy laid out the foundations of Calculus; based on a precise definition of the concept of
limit, he transformed the concepts of derivative and integral into the pillars of Calculus, leaving the
concept of differential relegated to a marginal role, useful to abbreviate certain formal expansions.
Even the definition of differential (dy = y’·dx) depended on the previous definition of derivative
(y’). This work, nevertheless, barely affected the use of Calculus in physical applications, where the
differential expressions continued to play an essential role, as starting points to tackle a great
number of problems and situations; not in vain, the Mathematics of the 19th century marks the
break up between Physics and Mathematics, considered by some as a divorce (Gonzalez-Urbaneja,
1991), and by others as a decolonization (Aghadiuno, 1992). It is not strange, then, that the
differential has kept in Physics the original meaning of Leibniz, without overcoming its
inconsistence, in spite of the precise definition of the concept of limit and of an infinitesimal
quantity provided by Cauchy2.
1
For example, approximating the portion of a sphere by the corresponding cylinder, the volume of the sphere is
correctly obtained, but not the area of its surface [Artigue and Viennot, 1987].
2
Cauchy defines an infinitely small quantity as “a variable whose numerical value decreases indefinitely so that it
converges towards the limit zero” [Cauchy, 1998, pp. 26-27]. It is obvious that the increment of any continuous
3. A definition which improves its use with comprehension in physics
At the beginning of the 20th century, in the context of Functional Analysis, the French
mathematician Fréchet formulated a new definition of differential, which “resembles the old
definition ... and offers all its advantages, but escaping all the objections of rigour that very justly
had been made to it”, according to a textual quote of the author [Artigue, 1989, p. 34]. This
definition is independent of the derivative, it is centred in the original idea of an approximation, and
provides a precise meaning to any differential expression; for this reason, we have used it –adapted
to functions of one variable- to rebuild the use of Differential Calculus in a wide variety of physical
situations, giving a precise answer to all the enigmas related with its comprehension (López-Gay et
al., 2001).
In short, dy (the differential with respect to x) is an estimate of the ∆y produced by an ∆x; such an
estimate is linear with respect to ∆x. Among all the possible linear estimates, the differential is the
one whose gradient (dy/dx) coincides with the derivative (y’); this guarantees the possibility of
obtaining the exact result by integration, that is, that the limit of the total error accumulated when
differentials are added is zero. This conception allows us to overcome the deficiencies related with
the original concept:
- It clearly shows that it is necessary to resort to the differential in non-linear situations and, for
this reason, the differential will never coincide with the increment (it can be bigger, or smaller),
no matter how small it may be.
- The differential is a new function of two variables: x, dx. Its value may be big or small,
depending on the value assigned to the variables x, dx. What is infinitely small, to a first order,
is the difference between differential and increment: lím
∆x → 0
∆y − dy
=0
∆x
-
The differential expressions acquire a precise meaning. For example, the expression: dp = F·dt
is used when the force is variable in the interval of time ∆t o dt, and is an estimate of how much
would the linear momentum change if the force remained constant during ∆t.
- Sometimes, it is easy to determine the differential expression: when the uniform behaviour is
known beforehand (for example, when the force is constant we know that: ∆p = F·∆t, therefore
the corresponding differential expression when it is not constant will be: dp = F·dt). But, in most
physical problems, that behaviour is not known beforehand, and the starting differential
expression must be advanced as a hypothesis, based on the physical analysis of the situation,
and with the only formal requirement of it being linear with respect to the change of variable.
The confirmation of that hypothesis can only be done through the result to which it leads.
This clarification not only leads to a better understanding of the nature and the significance of the
differential expressions, but also of the general strategy of Calculus to tackle a physical problem,
the reason for the steps that are taken when applying that strategy, and the meaning of other basic
concepts (derivative and integral) and the relationships among them. With this clarification we have
identified some indicators of what is an adequate comprehension of the differential in Physics:
1. To know when and why its use becomes necessary, that is, to know which is the problem that
makes ordinary calculus insufficient (i.e., it is necessary to resort to the differential when we
need the ∆y produced in an ∆x, and the relationship between both is not linear).
2. To understand the strategy that Calculus offers to solve that problem and comprehend the
reason of the different steps that are taken, i.e.:
a. To be able to explain with accuracy and physical sense the meaning of the expressions.
b. To know and justify the relationship that exists between the differential and the derivative y’
= dy/dx, and accept without ambiguity the reasoning in which this relationship is used.
c. To know the meaning of the integral and to know how to justify the so called Fundamental
Theorem, i.e., why the definite integral requires the calculation of antiderivatives or
primitive functions.
function obeys this definition, and therefore it is not useful to characterize the differential expression. Nor can one say
that it is the limit of the increment when it tends to zero as, if it is continuous, the differential would always be zero.
d. To use that strategy with full knowledge of the physical content in situations and problems.
3. To be aware of the hypothetical and exploratory nature, in almost all physical situations, of
the starting differential expression, and to know that the validity of that hypothesis cannot be
checked directly but through the result to which it leads.
4. To value positively the role of the differential in learning Physics. This axiological component
should be a natural consequence when the crucial role that the differential plays in the treatment
of physical situations of interest is understood.
These indicators have been of use to us as a guide to analyse the use of Calculus in the common
teaching practices, and to elaborate an alternative proposal.
4. Improving the use comprehension: first results
Although we are aware that it is arguable which is the adequate school year to introduce the use of
Calculus, we are convinced that, once that decision is made, it is necessary to do it right, without
expecting the comprehension to be acquired in the future, out of the blue. In our case, we begin the
use of Calculus in the 3rd year of BUP (16 years old) and complet it in the year of COU (17 years
old). We have designed programmes of activities for those two years based on the idea of the
differential as a linear estimation of the increment, paying attention to the indicators of an adequate
comprehension as listed above. Only in the 2nd year the concept of integral is introduced to all the
students, therefore we are not taking into account the indicators 2c and 2d in the 1st year. We have
carried out a detailed experimental design to confirm that the incorporation, from the beginning, of
the new proposal on the differential, improves the teaching and learning. The following diagram
shows the results of the analysis of a problem solved by students of COU, in which the use of
Differential Calculus was necessary. The students of the experimental group have had one of us
(RLG) as their teacher during two years.
We may mention also some results in relation with the attitude that students adopt. 63% of the
students of control groups (COU) do not pay attention when Differential Calculus is used in Physics
for they know they are not going to understand it and only pay attention to the final formula. This
clearly reflects the preponderance of the mechanicism in standard teaching practice. On the other
hand, 62% of the students of experimental groups (3rd of BUP) taught by teachers trained by us,
and 85.4% of the students of one us (RLG) (3rd of BUP), plainly reject this idea, which we interpret
as an indication of the use with comprehension which is promoted in their Physics lectures.
In general, the comparative analysis of the results obtained by experimental and control groups
show clearly that significant differences exist among them in all the indicators of an adequate
comprehension, always in favour of the experimental groups, whether of one of us or of teachers
trained by us. The differences become bigger with students who have used the new teaching
materials for two years. We find ourselves, then, in an adequate direction to improve the use with
comprehension of Differential Calculus in the teaching of Physics.
Control group COU (57)
Experimental group COU (18)
100
Percentage
80
60
40
20
0
Uses Differential He/She justifies it Some meaning to
Calculus
the differential
Writes integrals
Addition of a lot
of terms
Justifies
Fundamental
Theorem
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